1. Field of the Invention
The present invention relates to an image forming apparatus and a conveyance speed control method.
2. Description of the Related Art
There have been electrophotographic image forming apparatuses. An electrophotographic image forming apparatus forms a toner image by developing an electrostatic latent image formed on a photosensitive drum, transfers the formed toner image onto a paper sheet, and heats and fixes the transferred toner image, to form an image on the paper sheet.
When a long paper sheet (such as roll paper) is conveyed in an electrophotographic image forming apparatus, if a paper loop is formed between rollers provided in the conveyance path, the paper sheet is allowed to be in any position. As a result, paper wrinkling or meandering occurs. Therefore, the rollers provided closer to the downstream side of the conveyance path are designed to rotate at higher linear speeds, so that tension is applied to the paper sheet, and paper wrinkling and meandering are prevented.
When a printing process is performed in the above described image forming apparatus, a paper sheet is heated to a high temperature by the fixing device, and an image is fixed onto the paper sheet. Therefore, the heating conveyance roller of the fixing device thermally expands or contracts, and the outer diameter of the heating conveyance roller changes. During printing, for example, the temperature of the heating conveyance roller gradually increases, and thermal expansion occurs. The outer diameter of the heating conveyance roller becomes larger accordingly. As a result, the conveyance speed of the fixing device gradually becomes higher with respect to the conveyance speed of the transfer unit provided on the upstream side in the conveying direction.
As the conveyance speed of the fixing device becomes higher with respect to the conveyance speed of the transfer unit, the paper conveyance speed with respect to the intermediate transfer belt changes, the intermediate transfer belt is pulled by the paper sheet, and the speed of the intermediate transfer belt changes. The enlargement rate of the image to be printed then changes in the conveying direction, or color shifting then occurs due to a change in the transfer position of the image, resulting in lower print quality. Particularly, color shifting becomes larger as time passes after the start of the printing. Therefore, the degradation in quality becomes visible to users, and the commercial value of the printed material is seriously damaged.
The above problem can be solved by driving the heating conveyance roller at a speed in accordance with the outer diameter of the heating conveyance roller. The outer diameter of the heating conveyance roller depends on the temperature of the heating conveyance roller, and therefore, it is necessary to use a contact-type temperature sensor having higher measurement precision than a noncontact type.
As a technique using a contact-type temperature sensor, there is a disclosed temperature control technique by which a temperature sensor is provided at the center portion of a heating conveyance roller, the temperature of a portion at which a paper sheet exists is measured, and the temperature of the end portions of the heating conveyance roller is estimated based on information about the measured temperature, paper existence/nonexistence information, and size information (see JP 8-286551 A, for example).
There is also a disclosed technique by which temperature sensors are provided at the center portion and an end portion of the fixing belt in the width direction, temperatures are measured, the temperature of the heating conveyance roller is predicted based on analytical information about the temperature distribution, and the rotation drive speed is corrected based on the predicted temperature (see JP 2009-276580 A, for example).
However, a contact-type temperature sensor is in contact with the heating conveyance roller and damages the surface of the roller. Therefore, if such a contact-type temperature sensor is provided within the paper conveyance width as disclosed in JP 8-286551 A, the image formed on the back surface might be adversely affected.
By the technique disclosed in JP 2009-276580 A, contact-type temperature sensors are attached not to the heating conveyance roller but to the fixing belt. Therefore, the temperatures of the portions of the heating conveyance roller through which a paper sheet is actually conveyed cannot be accurately measured, and the paper sheet cannot be conveyed at a stable speed.